Molecular engineering of sterically-isolated spherical chromophores for high-performance second order nonlinear optical polymers

Four novel chromophores have been designed and synthesized, consisting of julolidine electron donors, tricyanofuran (TCF) or trifluoromethyl-phenyl-tricyanofuran (CF₃-Ph-TCF) as electron acceptors, and modified diene π-bridges. Based on the chromophores BHD-1 and BHD-2, benzene derivatives containing three 3,5-bis(trifluoromethyl) substituents were incorporated into π-electron bridge to obtain BHD-3 and BHD-4. Thermal stability, optical properties and microscopic nonlinearity calculated by density functional theory (DFT) of chromophores were examined to explore structure-property relationships. Macroscopic electro-optic (EO) activity was also evaluated. As the concentration of doped chromophores increases, the EO coefficient of BHD-4/PMMA becomes higher, reaching a maximum of 78 p.m./V. Compared with the host-guest system, the EO activity of the neat BHD-4 film without and with a titanium dioxide (TiO₂) charge-blocking layer have been significantly improved, particularly the latter achieving a maximum EO coefficient of 112 p.m./V. The results demonstrate that the introduction of 3,5-bis(trifluoromethyl) benzene derivatives as auxiliary isolation groups can modify the chromophore into a more spherical configuration, thereby enhancing EO performance.